This application is based upon and claims the benefit of priority of Japanese Patent Application No. 2000-325723 filed on Oct. 25, 2000, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a brake apparatus for a vehicle with ABS control function, in particular, applicable to an electric brake apparatus in which braking force is generated by driving electrically a power source such as a motor.
2. Description of Related Art
In ABS control of a conventional brake apparatus, as a brake pedal is stepped on, a braking actuator is driven to gradually increase brake torque as shown in FIG. 8 and, when a wheel speed becomes lower than a vehicle body speed, that is, when a slip ratio becomes larger than a given threshold value (target slip ratio), to rapidly reduce the brake torque so as to avoid a locking tendency.
However, since the brake torque is adjusted by driving the braking actuator for feedback control after the slip ratio exceeds the given threshold, the slip ratio is likely to become too large due to a response delay of the braking actuator. Accordingly, if smaller brake torque is applied to avoid the locking tendency, it will result in a lack of braking force.
Alternatively, it is known to ease an increasing gradient of the brake torque according to slip conditions so as to keep the slip ratio at a given value. However, since an absolute brake torque is unknown, a cycle that the brake torque increases until reaching lock and, after having reached the lock, decreases, will be ineffectively repeated.
An object of the invention is to provide a brake apparatus which generates effectively sufficient braking force during the ABS control.
To achieve the above object, in a vehicle brake apparatus for executing ABS control to prevent a wheel from locking in such a manner that braking force is controlled so as to keep a slip ratio of a wheel at a target slip ratio so that an excessive drop of a wheel speed from a vehicle body speed is adjusted, the apparatus has slip ratio calculation means for calculating a slip ratio based on vehicle body speed and wheel speed, a braking actuator for adjusting braking force applied to a wheel and a drive circuit for generating an output current by which the braking actuator is driven.
With the brake apparatus mentioned above, the output current during ABS control is set to a current on which oscillating waves are superimposed so that the wheel speed changes to show oscillating waves.
As mentioned above, the wheel speed changes in oscillating waves responsive to a change of the oscillating wave brake torque. Even if the pedal stroke amount of the brake pedal becomes larger and the wheel speed once drops largely below the vehicle body speed to an extent that the slip ratio almost exceeds the target slip ratio, further decrease of the wheel speed is restricted and the wheel speed turns to an increase soon since the wheel speed changes in the oscillating wave form and this cycle is repeated. Since the wheel speed automatically changes from decreasing to increasing irrelevant to the detection as to whether the slip ratio exceeds the target slip ratio, there is no excessive drop of the wheel speed due to the response delay, as the conventional brake apparatus does, so that sufficient braking force is generated.
It is preferable that the oscillating waves of the output current are formed in sine waves.
In more details, the brake apparatus has a braking actuator for adjusting braking force applied to a wheel, a drive circuit for generating an output current by which the braking actuator is driven, slip ratio calculation means for calculating the slip ratio based on the vehicle body speed land the wheel speed, first instruction current calculation means for calculating first instruction current corresponding to a depression state of a brake pedal, second instruction current calculation means for calculating second instruction current necessary for controlling braking force so as to keep the slip ratio at a vicinity of the target slip ratio once the slip ratio exceeds the target slip ratio during ABS control, comparison means for comparing the first instruction current with the second instruction current and generating third instruction current, which is set to the first instruction current when the second instruction current is larger than the first instruction current and to the second instruction current when the second instruction current is smaller than the first instruction current, and oscillating wave calculation means for generating fourth instruction current formed by superimposing oscillating waves on the third instruction current.
With the brake apparatus mentioned above, the fourth instruction current during ABS control corresponds to the output current by which the braking actuator is driven so that the wheel speed changes to show oscillating waves.
On detecting whether or not the wheel speed drops to an extent that the slip ratio exceeds the target slip ratio, it is preferable to use an integration value of the slip ratio, that is, an integration value of differences between the wheel speed and the vehicle body speed, a variation range of the wheel speed or a phase delay of the oscillating waves of the wheel speed from those of the output current.